Obesity is a nationwide epidemic and a leading cause of cardiovascular disease. Obese people display hypertension, impaired baroreflex control of arterial pressure (AP), and exaggerated pressor responses to stress, which contribute to end-organ injury and increased morbidity in obese patients. Altered sympathetic regulation of the heart and vasculature is integral to obesity-associated impairment of cardiovascular regulation, but mechanisms underlying deficits in sympathetic control are poorly understood. Obese Zucker rats (OZR) have autonomic deficits analogous to those observed in obese people: increased sympathetic nerve activity (SNA) with hypertension, blunted baroreflex-mediated changes in SNA, and exaggerated increases in SNA and AP with other sympatho-excitatory reflexes. Exaggerated sympatho-excitatory responses persist in the absence of baroreceptor feedback, suggesting additional baroreflex-independent alterations in the control of SNA in OZR. The opposing effects of obesity upon baroreflex versus other sympathetic reflexes are likely due to their disparate underlying mechanisms. Baroreflex-mediated increases in SNA are elicited by a withdrawal of GABAergic inhibition from the caudal ventrolateral medulla (CVLM) to the brainstem neurons that drive SNA in rostral ventrolateral medulla (RVLM). In contrast, other sympatho-excitatory stimuli raise SNA by glutamatergic or angiotensinergic stimulation of the RVLM. We hypothesize that OZR have a dual deficit in sympathetic regulation of cardiovascular function: impaired baroreflex-mediated GABAergic inhibition of the RVLM, AND enhanced sensitivity of RVLM neurons to excitatory stimuli controlling sympathetic vasomotor tone. In Aim 1 we will determine if impaired baroreflexes are due to deficits in baroreceptor afferent function or changes in the brain stem. In Aim 2 we will determine if OZR have a reduced GABAergic inhibition of the RVLM. In Aim 3 we will determine whether excitation of the RVLM with glutamate or angtiotensin II produces larger increases in SNA and AP in OZR, even without baroreflexes. In Aim 4 we will determine whether OZR also have exaggerated sympatho- excitatory responses initiated by the forebrain, which activate SNA exciting the RVLM. This proposal will use state-of-the-art anatomical and electrophysiological measures to provide the first mechanistic understanding of deleterious changes in brain stem control of autonomic regulation associated with obesity.